Throughout human history, beneficial microbes have adapted to thrive on specific substances in the food we eat. Yet only in the past few decades have scientists homed in on which substrates —and underlying mechanisms of action—are used to best nourish them.
The concept of “prebiotics” has since expanded into a new category with heightened importance as it becomes clear that our microbial ecosystems impact health across our lifetimes.
Prebiotics, in brief
Diet—and now supplements—are the chief source of energy for the growth of gut microbiota. Particularly, non-digestible carbohydrates can highly modify the composition and function of gut microbiota. Beneficial intestinal microbes ferment these non-digestible dietary substances and obtain their survival energy from degrading indigestible bonds of prebiotics. As a result of this, prebiotics can selectively influence gut microbiota. We truly are what we eat.
Prebiotics, a definition
Prebiotics were first defined in a 1995 publication called “Dietary modulation of the human colonic microbiota: Introducing the concept of prebiotics” and their definition has continued to evolve over time although the debate is still ongoing.
The current definition of a prebiotic was developed by a panel of experts convened by the International Scientific Association for Probiotics and Prebiotics (ISAPP) in 2016.
Prebiotic: “a substrate that is selectively utilized by host microorganisms conferring a health benefit.”
To some degree, scientific and regulatory definitions of prebiotics differ, although the health benefits of these compounds are uniformly agreed upon to be due to their fermentability by gut microbiota.
- Increases in Bifidobacteria and Lactobacilli, bacteria highly associated with health.
- Fermentation of prebiotics by gut microbiota produces many beneficial metabolites including lactic acid and short-chain fatty acids (SCFAs), including acetate, butyric acid, and propionic acid which are linked to health in various ways.
- Increases in calcium absorption, which can prevent or treat bone diseases
- Reduction in protein fermentation, a process that can produce harmful metabolites
- Decreases in pathogenic bacteria populations
- Decreases in allergy risk
- Effects on gut barrier permeability, promoting tight junctions and less inflammation
- Improved immune system defense, in many instances from metabolites such as SCFAs.
- Positive effects on insulin resistance and blood sugar levels
Notably, prebiotics are able to affect not only the gastrointestinal tract (conditions such as irritable bowel syndrome and inflammatory bowel diseases) but also other distant site organs and systems.
Sources of prebiotics
Some plant fibers qualify as prebiotics. Importantly, as the definition states, the beneficial effect bestowed on the host by the prebiotic should result from specific resident microbes utilizing that prebiotic. Other compounds such as polyphenols also meet the requirements to be called prebiotics.
Types of prebiotics
- Fructans, which include inulin and fructo-oligosaccharide (FOS)
- Galacto-oligosaccharides (GOS), some of which are derived from lactulose, the isomer of lactose.
- Modified starches, particularly resistant starches which produce butyrate.
- Non-carbohydrate oligosaccharides including flavanols, a type of polyphenol with prebiotic benefit
- Human milk oligosaccharides (HMOs) enhance beneficial microbial populations in the infant gut. Oligosaccharide prebiotics are often added to infant formula.
The list of common prebiotic foods includes the following foods, with the average amounts of inulin and FOS in a 100-gram portion.
- Chicory root: 41.6 g inulin and 22.9 g FOS
- Jerusalem artichoke: 18.0 g inulin and 13.5 g FOS
- Dandelion greens: 13.5 g inulin and 10.8 g FOS
- Garlic: 12.5 g inulin and 5.0 g FOS
- Leeks: 6.5 g inulin and 5.2 g FOS
- Asparagus: 2.5 g inulin and 2.5 g FOS
- Wheat bran: 2.5 g inulin and 2.5 g FOS
- Baked wheat flour: 2.4 g inulin and 2.4 g FOS
- Banana: 0.5 g inulin and 0.5 g FOS
Prebiotics may also be added to probiotic supplements as well as foods and beverages. The label should identify ingredients such as galacto-oligosaccharides (GOS), fructo-oligosaccharides (FOS), chicory fiber, or inulin.
What’s the right amount of prebiotics to consume?
There is no official dietary recommendation for the intake of prebiotics. ISAPP suggests the following: “Typically, around 5 grams is the target for FOS and GOS in the daily diet—and this includes dietary sources of prebiotics.” Be aware that this is not the same as the daily dietary fiber recommendation, which is 28 g/day, based on a 2000 kcal/day diet.
A well-chosen diet can provide much of the prebiotic need and supplements can make up the slack. As with other dietary compounds, moderation and variety are key in usage. But complicating matters, emergent research suggests that gut microbial responses to prebiotic interventions vary by individual. Prebiotic intake leads to varying effects on SCFA production in human studies. In the future, personalized recommendations may be possible.
When selectively utilized by host beneficial microorganisms, prebiotics can positively influence the microbiota and affect health and wellness. Researchers have isolated many of these prebiotics along with a clearer view of their mechanisms of action.
A healthy diet can provide for many of our needs while isolated prebiotic ingredients added to probiotics supplements, foods, and beverages can boost intake.
Future research will help define specific needs and individual responses.
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